1. Field of the Invention
The present invention relates to a zoom lens, which is suitably used for an image pickup apparatus using a solid-state image pickup element, such as a digital still camera, a video camera, a television camera, or a surveillance camera.
2. Description of the Related Art
In a camera with interchangeable lenses, a camera having a function of displaying image data, generated by an image pickup lens and an image pickup element, on a display element (a live view function) in accordance with computerization of the image pickup element is known. In these cameras with the interchangeable lenses, a mirrorless camera that removes an optical finder using a quick return mirror, a pentaprism capable, and the like, and that is capable of confirming a shooting image using the live view function described above is known. Since this mirrorless camera removes the quick return mirror, the thickness of the camera is easily reduced and the size of entire of the camera is easily reduced.
On the other hand, an autofocus (AF) device is mounted on the camera in many cases. In a lot of cameras with the quick return mirrors, a ranging member that is used to perform an autofocus operation using a phase difference method (hereinafter, referred to as a “phase difference AF”) is stored in a mirror box. However, since the mirrorless camera does not have the mirror box, there is little space for placing the ranging mirror used for the phase difference AF, and therefore it is very difficult to place the ranging member used for the phase difference AF without increasing the thickness of the camera.
When the phase difference AF of a twin-lens external method is adopted, it is difficult to perform the phase difference AF in a wide shooting range over the entirety of an object distance from an infinite object to a macro range as a zoom range from a wide angle end to a telephoto end. Therefore, as the AF of these mirrorless cameras with interchangeable lenses, a contrast detection AF in which an in-focus operation is performed based on the contrast of image data outputted from the image pickup element is adopted in many cases.
In the contrast detection AF, a focus lens unit is driven with vibration at high speed (for example, around 30 frames/sec so as not to have uncomfortable feelings during the in-focus operation) in an optical axis direction, and an in-focus position is calculated based on changes of the contrast of the image (wobbling operation). Therefore, in the image pickup lens which can handle the contrast detection AF, the focus lens unit is required to be small in size and weight.
As a zoom lens having a small-size total system and easily achieving a wide angle of view, a negative lead type zoom lens in which a lens unit having a negative refractive power is positioned at an object side is known. Furthermore, as a negative lead type zoom lens with a wide angle of view, a four-unit zoom lens that is configured by, in order from an object side to an image side, lens units having negative, positive, negative, and positive refractive powers, and that changes an interval between adjacent lens units so as to perform a zooming operation is known.
In this four-unit zoom lens, in order to realize reduction in size and weight of the focus lens unit, a four-unit zoom lens in which the focus lens unit is disposed in other than a first lens unit so as to use an inner focus method is known. U.S. Pat. No. 7,777,967 discloses a four-unit zoom lens which performs a focusing operation using a fourth lens unit. Each of Japanese Patent Laid-Open Nos. 2003-131130 and 2001-343584 discloses a four-unit zoom lens which performs the focusing operation using a third lens unit.
In the negative lead type zoom lens, when a desired zoom ratio and reduction in size of the total lens system are achieved, a retro focus type power arrangement at the wide angle end and a telephoto type power arrangement at the telephoto end are applied. Therefore, the total lens length at the wide angle end is longer than the total lens length at the telephoto end.
As a negative lead type zoom lens, in order to shorten the total lens length at the wide angle end, there is a three-unit zoom lens that is configured by lens units having negative, positive, and negative refractive powers, and that has a lens unit having a negative refractive power positioned in a final lens unit. The three-unit zoom lens has an exit pupil position of an optical system which comes close to an image plane by the effect of the negative refractive power of the final lens unit. Therefore, an incident angle of a ray onto the image pickup element is increased, and therefore a large amount of shading is generated when using an electronic image pickup element.
On the other hand, the four-unit zoom lens described above easily achieves the wide angle of view and the reduction in size of the total system, and has characteristics that has a little generation of the shading since the incident angle of the ray onto the image pickup element is decreased by the function of the positive refractive power of the fourth lens unit. However, in order to reduce the amount of the shading and perform a high-speed focusing operation using lens units of the small-size and lightweight total lens system while achieving the wide angle of view, it is important to appropriately set a refractive power (a power) of each lens unit, a lens configuration, and the like.
For example, if the lens configuration of the first lens unit or the third lens unit, the refractive power of the second, third, or fourth lens unit, or the like is not appropriately set, it is difficult to have a high optical performance over the entirety of the zoom range at the wide angle of view while achieving the reduction in size of the total system so as to perform the high-speed focusing operation.
In an interchangeable zoom lens used for the camera system with interchangeable lenses, when a lens unit disposed closest to the image side is moved during the focusing operation, an interference of an actuator that moves the lens unit disposed closest to the image side with a mount member is a problem.